Process for regenerating spent photographic silver bleaching solutions

ABSTRACT

The alkali metal ferricyanide and alkali metal salt in a bleaching solution for bleaching the silver developed in a color photographic material is regenerated by continually producing a suitable free halogen in a separate vessel and for delivery to the bleaching bath. The quantity of halogen produced is in direct proportion to the need for regenerating halogen as determined by the measurement of the ratio of ferricyanide to ferrocyanide in the bleaching bath. The ratio is measured by means fo the redox potential of the bleaching bath which controls the rate of feeding of reactants to the halogen-forming reaction in the vessel. The free halogen is fed into the bleaching bath under the surface. Other baths for processing the photographic material are regenerated by variably controlling the input of the respective regenerating substances into the respective baths by means of the redox potential.

United States Patent 1 Hiinicke et al.

Oct. 1, 1974 PROCESS FOR REGENERATING SPENT PHOTOGRAPHIC SILVERBLEACHING Primary Examiner-Ronald H. Smith SOLUTIONS AssistantExaminer-M. F. Kelley [75] Inventors: Wolfgang Hiinicke, Leverkusen;Attorney Agent or Flrm connouy and Hutz Karl-Wilhelm Schranz,

Odenthal-l-I ahnepberg, both of 7 Assignee; A(;FA Gevaert Akti ll h ftThe alkali metal ferricyanide and alkali metal salt in a LeverkusemGermany bleaching solution for bleaching the silver developed in a colorphotographic material is regenerated by [22] Fled: 1973 continuallyproducing a suitable free halogen in a sep- 21 APPL 353 101 arate vesseland for delivery to the bleaching bath. The quantity of halogen producedis in direct proportion to the need for regenerating halogen as deter-[30] Apphcatlon Pnomy Data mined by the measurement of the ratio offerricyanide Apr. 27, 1972 Germany 2220667 to ferrocyanide in thebleaching bath. The ratio is measured by means to the redox potential ofthe [52] US. Cl. 96/50 A, 96/60 R bleaching bath which controls the rateof feeding of [51] Int. Cl G03c 5/32, G03c 5/26 reactants to thehalogen-forming reaction in the ves- [58] Field of Search 96/50 A, 60 Rs l,

The free halogen is fed into the bleaching bath under [56] Referencesand the surface. Other baths for processing the UNITED STATES PATENTSphotographic material are regenerated by variably 7 2,515,930 7/1950Seary 96/50 A controlling the input of the respective regenerating2,611,699 9/1952 Zappert substances into the respective baths by meansof the 3,615,507 Bard BF redox potential 3,770,437 11/1973 Brugger etal. 96/60 R T878907 9/ 1970 Walsh 96/60 R 8 Claims, 4 Drawing Figures 1Q1 r I 1 I I I 7 ,1 z A "2 I 7 215 A -2/5 220 r 2 I 1 237 204 1 W [J 1214 r l I f l I 1 I I l I I I anew-43 PAIENIEU W 1 974 sum 10F 2 FIG. 7

PAIENTEDUBI 1 I974 sum 2 OF 2 FIG. 4

44 /1 K3 Fee/v PROCESS FOR REGENERATING SPENT PHOTOGRAPHIC SILVERBLEACHING SOLUTIONS This invention relates to a process for theregeneration of spent bleaching baths which contain mainly a solublealkali metal ferricyanide and an alkali metal halide.

Bleaching baths are used in photographic processes for converting thesilver formed on development into a salt which is soluble in the fixingbath. The bleaching baths generally used contain an alkali metalferricyanide, an alkali metal halide and other additives such as buffersto adjust the pH, anti-corrosives, watersofteners and bleachingaccelerators.

When these solutions are used for bleaching the silver developed in thecolor photographic material, the ferricyanide is reduced to ferrocyanideand the halide ions are bound by the oxidized silver as follows:

The efficiency of the bleaching solution therefore diminishes withprogressive use so that the exhausted solution must either be discardedor regenerated. It is known that the ferrocyanide can be converted intoferricyanide by the addition of halogens to the exhausted bleachingbath. At the same time, halide ions are formed which compensate for thelosses which occur in accordance with equation (I) by the followingreaction:

Reoxidation of the ferrocyanide with anhydrous halogens as describede.g. in US. Pat. No. 2,515,930, however, has so far not become generallyestablished in practice because. the necessary compounds such aschlorine and bromine are very dangerous substances which are difficultto handle.

According to German Patent Specification No 866,909, the bleaching bathis reoxidized by the addition of an alkali metal hypobromite or amixture of an alkali metal bromate and hydrobromic acid or a mixture ofalkali metal bromates and alkali metal bromides with the addition ofalkali metal bisulfates, sulfamic acid or sulfuric acid. As is clearfrom the description and examples, the alkali metal bromate or mixtureof alkali metal bromate and alkali metal bromide used in this process isdissolved in the bleaching bath and hydrobromic acid or sulfuric acid isthen added. If the mixture which liberates bromine is mixed with acidbefore addition to the bleaching bath, bromine is liberatedspontaneously, which causes serious inconvenience.

The process has the disadvantage that considerable dilution takes placewhen the reactant which liberates bromine is dissolved in the bleachingbath, with the result that the reaction velocity is very low owing tothe low concentractions. The pH of the bleaching bath is greatly reducedby hydrobromic acid or sulfuric acid which has not yet reacted, andalthough the reoxidation velocity is increased, the decompositionvelocity of the bleaching bath is also greatly increased. Quantitativereaction of the reactants added to the bleaching bath is not achievedwithin an economical time since the concentrations continue to decreasewhile the pH rises as the reaction progresses. Repeated adjustment ofthe pH, which has been suggested for sodium hypobromide, is complicatedand time-consuming and results in an undesirable increase in the saltconcentrations.

It is an object of this invention to find a simple and economicalprocess for regenerating or reoxidizing spent ferricyanide bleachingsolutions. It is important, especially with a view to reducing theferricyanide concentrations in effluent water, that the bleaching bathshould not accumulate unwanted salts such as sulfates in the reoxidationcycle because the whole bleaching bath or large quantities of it wouldhave to be discarded owing to the rapid decrease in activity. It is alsodesirable to forego adjusting the bleaching bath to very low pl-l-valuesbecause the stability of the bath would thereby be impaired.

This invention relates to a process for the regeneration of spentphotographic silver bleaching baths which contain alkali metalferricyanide and alkali metal halide by reoxidizing the ferrocyanidesformed in the process with bromine, characterised in that a quantity ofbromine equivalent to the concentration of ferrocyanide at any givenmoment is continuously produced in aqueous solution in a closed vessel,controlled by the redox potential of the bleaching bath, and flows inunder the surface of the bleaching bath. In this way, bromine is onlysurface of the bleaching bath. In this way, bromine is only produced insmall quantities at a time in a closed vessel and therefore cannotescape into the environment.

Numerous reactions are known for producing halogens. They are mainlybased on the oxidation of halide ions in acid aqueous solutions, and insome cases compounds of the given halogens in a higher valency state areused as the oxidizing agents. The last mentioned reactions are alsoknown as synproportionating reactions.

Liberation of halogens may be achieved, for example, in accordance withthe following reaction equations.

KBrO SKBr 6HCl 3Br 6KCl 3H O H202 Bl'z l rvn KOBr ZHBr Br KBr H O (Vll)K10 SKI 6HCl 3l 6KCI 3H O (Vlll) The reactions according to equations(III) to (VIII) are only a selection from the possible oxidationreactions. They have in common the fact that the formation of thehalogen is not accompanied by the formation of any reaction productswhich could reduce the bleaching activity of the bath. The alkali metalsalts formed are constituents of the bleaching baths and therefore donot interfere with bleaching even at fairly high concentrations. Sinceprocessing of the photographic materials is inevitably accompanied by acertain amount of dilution of the bleaching bath due to liquid beingcarried into or out of the bath by the photographic material, anequilibrium of salt concentraction becomes established. The equilibriumconcentration may be so low that further addition of alkali metalhalides or alkali metal phosphates to the bleaching bath becomesnecessary.

The reactions according to equations (VII) and (VIII) have not achievedany position of importance in general practice since (equation VII)potassium hypobromite must first be prepared by a complicated reactionbetween bromine and KOH and moreover is an unstable compound and since(equation VIII) iodine used in the bleaching process forms silver iodidewhich is fixed much more slowly than silver bromide.

Equations (III) to (VI) on the other hand, are suitable for obtainingaqueous bromine solutions in accordance with the present invention. Inall cases, the reaction depends on the presence of acid which is usedup. If the reaction is fully completed, neutral solutions are obtainedif the reactants are mixed in the molar ratios indicated by theequations. According to equations (III) to (VI), bromide ions areoxidised in an acid solution; in equations (III) to (V) bromate is usedas oxidising agent and the reaction is therefore a synproportionatingreaction.

Reference will now be made to the accompanying drawings in which FIG. 1shows a reactor for producing the aqueous bromine solution by theprocess according to the invention;

FIG. 2 shows a practical example of reoxidation apparatus for theprocess according to the invention;

FIG. 3 is a schematic representation of the control and dosing pumpassembly for the process according to the invention;

FIG. 4 is a diagram representing the dependence of the redox potentialon the ratio of ferricyanide to ferrocyanide concentration in ableaching bath.

Redox potential is an abbreviation for reductionoxidation potential andis understood hereinafter to mean the electric potential between aplatinum electrode dipped into the solution under consideration and asecond reference electrode. This second reference electrode is for thepurpose of defining the redox potential a socalled standard hydrogenelectrode, which is represented by a platinum electrode charged withhydrogen at one atmosphere and dipping into a hydrogen ion solutionhaving a hydrogen ion activity of 1. More convenient are other referenceelectrodes such as the calomel electrode or the Ag/AgCl electrode. Theredox potential is an indication of the oxidizing power and thereforindirectly of the concentrations of the oxidized and reduced forms in aredox system.

According to the invention, the starting materials, which are easy tohandle in the form of aqueous solutions, are mixed in a small glassapparatus represented in FIG. 1. In this apparatus, oxidation of thehalide ions takes place instantly in acid solution with formation of thefree halogen. By using high concentrations of starting materials, suchhigh reaction velocities are usually obtained at room temperature that ahigh capacity is achieved even with a small apparatus.

Moreover, the volumes required are so small when concentrated solutionsare used that in the process according to the invention practically noincrease in volume occurs in the processing tank. It is thereforepossible to reduce the amount of effluent water by more than percent.

The reactor represented in FIG. 1 substantially comprises two inlettubes 101 and 102, a reaction chamber 103 and an outlet tube 104. Thereaction chamber may be filled with small filling bodies 15 such asRaschig rings, glass spirals or glass pellets. A solution of the acid,for example hydrohalic acid, is introduced through the inlet tube 101and a solution of oxidizing agent through the other inlet tube 102. Thetwo solutions meet at 106 where vigorous mixing occurs. The reaction,which sets in rapidly, takes place mainly in the reaction chamber 103.The length of the flow path of the reactants is considerably increasedby the filling bodies 105 and unwanted circulation of the solution dueto the thermal flow and the injection effect of the solution enteringfrom 106 are effectively suppressed. After an average time of stay ofthe solution in the reaction chamber 103 which is equal to the quotientof the volume of the reactor and the quantity passing through it perunit time, the liquid is discharged through the outlet tube 104.According to the invention, the outlet tube 104 is dipped into the spentbleaching bath, and the halogen which is discharged from it in the formof an aqueous solution is greedily taken up by the spent bleaching bath.The ferrocyanide is thereby converted into the required ferricyanide bythe reaction represented in equation II and at the same time the haliderequired for the reaction according to equation 1 is formed from thehalogen.

FIG. 2 shows one embodiment of the reoxidation apparatus according tothe invention. It consists of the closed vessel 207 with inlet tube 208for spent bleaching bath and outlet tube 209 for reoxidized bleachingbath. The reactor (FIG. 1) is fixed inside the sealed vessel 207. Thereactor is equipped with non-return valves 210, 211 and 212 whichprevent entry of the bleaching bath from the closed vessel 207 into thereaction chamber 203 and prevent escape of free halogen from thereaction chamber 203 into the surroundings by way of the inlet tubes 201and 202.

A fast stream of spent bleaching bath is directed through the inlet tube208 to the tip of the glass tube 204 (outlet tube of the reactor) sothat aqueous solution of halogen leaving the outlet tube is immediatelytaken up by the bleaching bath. After a certain time of stay, thebleaching bath passes over the darn 213 which has a safety aperture 237to the outlet tube 209 from where it can be conducted into theprocessing tank. Delivery of the bleaching bath may be carried out withthe aid of any liquid pump which must, however, be resistant toferricyanide. The state of oxidation of the bleaching bath is determinedwhenever required by means of a redox potential measuring chain whichmay consist, for example, of a platinum electrode 214 as measuringelectrode and a silver/silver chloride electrode 215 as referenceelectrode. Whereas the platinum electrode 214 dips directly into thebleaching bath, the silver chloride electrode 215 is placed in apotassium chloride solution 216 inside the inner tube 222 which issurrounded by yet another protective tube 218 filled with potassiumchloride solution 217, and electrically connected to the bleaching bathby way of the diaphragms 220 and 221.

The function of the protective tube 218 is to prevent contamination ofthe silver/silver chloride electrode by bleaching bath entering throughdiaphragm 220. For this purpose, the tube 218 is filled with potassiumchloride solution 216 right up to the storage bulb 219 so that there isa slight flow of potassium chloride 216 from the inner tube 222 into theprotective tube 218 due to the hydrostatic pressure on the diaphragm220. Any bleaching bath penetrating diaphragm 221 to enter theprotective tube 218 can be removed through the overflow 223. A smallsafety aperture 237 in the dam 213 prevents an overdose of free halogeninside the housing 207 in the event of failure of the delivery ofbleaching bath. Halogen discharged from the outlet tube 204 after ashort time reaches the measuring electrode 214 through the aperture 237,whereby the dosing apparatus 331 (FIG. 3) is switched off immediately.The voltage applied to the electrode leads 224 and 225 is measured by anelectric control instrument which forms part of a control and dosingpump assembly.

FIG. 3 is a schematic representation of such an assembly consisting ofcontrol instrument 326 and dosing unit 331. The control instrument 326consists substantially of a direct current amplifier 327 and an mV-measuring instrument 328 with a lower and upper limit voltage contact329 and 330. Connected to this is a dosing unit 331 which contains acommon drive motor 332 for the two dosing pumps 333 and 334. The pumps333 and 334 are connected at the input end to the storage tank forhydrohalic acid 335 and storage tank for oxidising agent 336 by way offlexible tubes while the pump output ends are connected to the inlettubes 301 and 302 of the reoxidation apparatus.

In the reactions according to equations (III) to (VII) it is importantto keep the ratio in which the starting materials are mixed constant.Pumps with accurately reproducible delivery rates should therefore beused for dosing the solutions. Gear wheel pumps, hose pumps, pistonpumps and some types of membrane pumps have been been found in practiceto be suitable for this purpose. The simplest way of keeping the ratioof delivery rates of the two pumps constant is to use a common drive.

The threshold contact 330 is adjusted to the redox potential whichcorresponds to the required ratio of potassium ferricyanideconcentration to potassium ferrocyanide concentration. The dosingapparatus continues to deliver hydrohalic acid and oxidizing agent untilthe potential on the platinum electrode 214 reaches the adjusted redoxpotential, and the motor 332 is then switched off at the contact 330.When bleaching developed filmed material, ferricyanide is reduced toferrocyanide and the redox potential drops. When the redox potentialfalls below the level to which the contact 330 is adjusted, the motor332 is switched on again. For the sake of achieving uniform control andhence constancy of operating conditions, it may be advantageous toadjust the delivery rate of the dosing pumps 333 and 334 to the rate ofbleaching of the processing machine. The best way to achieve this is toconnect the reoxidation apparatus to the pumping cycle of the bleachingbath tank so that the ferrocyanide concentration of the processing tankis controlled virtually continuously and kept constant. The thresholdcontact 329 can be used most simply for signalling when the potassiumferrocyanide concentration exceeds a previously adjusted maximum level.Furthermore, a relay may be connected to the threshold contact 329 toact as cable break safety fuse for the lead to the detecting element sothat the dosing apparatus 331 will be switched off as soon as the redoxpotential drops below the value to which contact 329 is adjusted.

FIG. 4 is a diagram showing the relationship between the redox potentialand the ratio of potassium ferricyanide concentration to potassiumferrocyanide concentration in a bleaching bath (formulation see example1). It will be seen that the rate of change of redox potential increaseswith decreasing proportion of ferrocyanide. In the region of lowferrocyanide concentrations, the control circuit has a high sensitivityto changes in ferrocyanide concentration. Since in practice it isdesirable to have only low ferrocyanide concentrations in bleachingbaths, the corresponding redox potential range is eminently suitable forcontrolling a reoxidation system.

The process according to the invention may advantageously be used as athrough-flow process for continuously regenerating spent bleaching bathsby reoxidation of the ferrocyanide with bromine. Local accumulation ofbromine which, as is well known, causes decomposition phenomena, iseffectively prevented by the fact that the outlet tube 204 is situatedin a region of vigorous flow so that any bromine discharged from it isimmediately diluted or used up in oxidizing the ferrocyanide. The redoxpotential measured by electrometric measuring chains in the bleachingbath can be used not only for controlling the regeneration of thebleaching bath but also for indirectly controlling the regeneration ofother processing baths, e.g. a developing, fixing, buffering, hardeningor stabilizing bath. This applies particularly to baths in which theactive substance is used up at the same rate as the silver is bleachedin the bleaching bath, e.g. the color developer bath preceding thebleaching bath. As is well known, the quantity of color developeroxidized in the color developer bath is proportional to the quantity ofdevelopable silver halide and hence to the quantity of elementary silverformed. The concentration of color developer substance in this bathdecreases accordingly. In the bleaching bath which follows the colordeveloper bath the silver reduces a proportional quantity of potassiumferricyanide. The quantity of regenerating solution required forregenerating the color developer bath is therefore proportional to thequantity of bromine required for regenerating the bleaching bath.

When the redox potential in the bleaching bath sinks, then 1, theconcentration of ferrocyanide in the bleaching bath increases inproportion to the conversion of silver into silver halide, and

2. the concentration of colour developer in the developer bath decreasesin proportion to the formation of dye and image silver.

All that is therefore required is to connect an additional dosing pumpfor the reactant which regenerates developer to the dosing assembly 331,which pump is switched on and off simultaneously with the dosing pumps333 and 334.

The time lag between the bleaching of a certain silver image andthepreceding development of the same image results in a correspondingidle time in the control circuit as regards regeneration of the colordeveloper bath. Since, however, the buffering capacity of the developerfor color developer substance is relatively high and moreover theaverage quantity of silver per unit surface area of the photographicmaterial is substantially constant, this idling time does not inpractice give rise to any trouble.

In most photographic processing baths the quantity of regeneratingsolution required for regeneration is proportional to the quantity ofphotographic material passing through the baths and hence to the totalquantity of silver halide whereas in the color developer bath and thefollowing bleaching bath the consumption and regeneration depend only onthe quantity of exposed silver halide. It has been found, however, thaton an average the quantity of silver halide which has been exposed andcan be developed by color development bears an almost constant ratio tothe total quantity of silver halide and that this ratio is approximately1:3. It follows from this that the other processing baths can beregenerated in the same manner, controlled by the electrometricmeasurement of the redox potential in the silver bleaching bath. Thisapplies particularly to baths in which the concentration of activesubstance need not be observed strictly accurately, e.g. the finalfixing baths, buffering baths, hardening baths and stabilizing baths.For these baths, regeneration controlled by the redox potential in thebleaching bath constitutes a simple and economical methods.

EXAMPLE 1 200 metres of a color negative film 35 mm in width which hadbeen exposed imagewise and then color developed were bleached in 10litres of a ferricyanide bleaching bath of the following composition:

potassium ferricyanidc 44 g pot ium ferrocyanide l g potassium bromide13 g potassium dihydrogen phosphate 17 g disodium hydrogen phosphate 7 gsodium hexametaphosphate 7 g made up with water to 1000 cc Analysis ofthe bath after bleaching showed that the ferricyanide concentration haddropped to 38.2 g/l and the potassium bromide concentration to 10.7 g/lwhile the ferrocyanide concentration had risen to 5.3 g/l. The pH of thebath was found to be 5.9. The bath is shown in FIG. 2 for reoxidation ofthe exhausted bleaching solution. The threshold contact 330 (FIG. 3) wasadjusted to a redox potential of 335 mV corresponding to the desiredpottassium ferrocyanide concentration of 1 g/l as represented in thediagram in FIG. 4. 71 cc of a 5.6 percent potassium bromate solution and71 cc of a 14.5 percent hydrobromic acid solution were pumped into theinlet tubes 201 and 202, respectively, of the reoxidation apparatus fromthe dosing apparatus 331 within a few minutes. By that time, the redoxpotential had increased to 335 mV and delivery of potassium bromatesolution and hydrobromic acid stopped. Fresh analysis of the bath showedthat the ferricyanide concentration had risen to 42.1 g/l and thepotassium bromide concentration to 12.5 g/l while the ferrocyanideconcentration had dropped to 0.92 g/l. The pH of the bath remainedunchanged at 5.9.

Loss of concentration due to material being carried along with the filmpassing through the apparatus was made up by the addition of 2 g/l ofpotassium ferricyanide and 0.5 g/l of potassium bromide.

Photographic tests showed that the bleaching effect of the reoxidisedand regenerated bleaching bath was equal to that of the fresh solution.

EXAMPLE 2 10 litres of an exhausted bleaching bath have the samecomposition as in example 1 were reoxidized and regenerated. 72 ml ofa5.6 percent potassium bromate solution and 72 ml of a solutioncontaining 200 g/l of potassium bromide and 204 g/l of a 36 percenthydrochloric acid were pumped as reoxidation solutions into the inlettubes 201 and 202 of the reoxidation apparatus (FIG. 2). The redoxpotential rose to 335 mV and the supply of reoxidation solutionsstopped.

Analysis of the bath indicated the presence of 42.0 g/l of potassiumferricyanide, 122 g/l of potassium bromide, 0.6 g/l of potassiumchloride and 1.0 g/l of potassium ferrocyanide and a pH of 5.9. 2 g/l ofpotassium ferricyanide and 0.8 g/l of potassium bromide were added tomake up the concentration.

The bleaching effect of the reoxidised and regenerated bleaching bathwas excellent.

EXAMPLE 3 10 litres of an exhausted bleaching bath of the samecomposition as in example 1 were reoxidized and regenerated. 72.5 ml ofa 5.6 percent potassium bromate solution and 72.5 ml of a solutioncontaining 410 g/l potassium bromide and 230 g/l of an percentorthophosphoric acid were pumped as reoxidation solutions into the inlettubes 201 and 202 of the reoxidation apparatus according to FIG. 2. Theredox potential rose to 335 mV and supply of the solutions stopped.

Analysis indicated 42.1 g/l of potassium ferricyanide, 1.08 g/l ofpotassium ferrocyanide and 12.94 g/l of potassium bromide. The pH of thebath remained practically unchanged at 5.85. Only 2 g/l of potassiumferricyanide were added to make up the concentration.

The bleaching effect of the reoxidized and regenerated bleaching'bathwas excellent.

EXAMPLE 4 10 litres of an exhausted bleaching bath having the samecomposition as in example 1 were reoxidized and regenerated. 24 ml of a10 percent hydrogen peroxide solution and 16 ml of a 48 percenthydrobromic acid solution were pumped as reoxidation solutions into theinlet tubes 201 and 202 of the reoxidation apparatus according to FIG.2.

Analysis of the bath indicated 41.8 g/l of potassium ferricyanide, 1.1g/l of potassium ferrocyanide, 12.4 g/l of potassium bromide and a pH of5.9. 2 g/l of potassium ferricyanide and 0.6 g/l of potassium bromidewere added to make up the concentration. The bleaching effect of thereoxidized and regenerated bleaching bath was excellent.

EXAMPLE 5 metres of a color reversal film 35 mm in width which had beenexposed imagewise and then developed were bleached in 10 litres of aferricyanide bleaching bath of the following composition:

Potassium ferricyanide l potassium ferrocyanide potassium bromidedisodium hydrogen phosphate made up with water to pH adjusted withacetic acid to Analysis of the bath after bleaching showed that theferricyanide concentration had dropped to 86.9 g/l and the potassiumbromide concentration to 15.0 g/l while the ferrocyanide concentrationhad risen to 12.5 g/l. The pH of the bath was found to be 5.2. The spentbleaching bath was reoxidized and regenerated in example l.

The threshold contact 330 was adjusted to a redox potential of 350 mV(redox potential of unused bath). 168 ml of a 5.6 percent potassiumbromate solution and 168 ml of a 14.5 percent hydrobromic acid werepumped into the inlet tubes 201 and 202 of the reoxidation apparatuswithin a few minutes. The redox potential had then risen to 350 mV andthe dosing pumps were automatically switched off. Analysis of the bathindicated 95.1 g/] of ferricyanide, 1.3 g/l of ferrocyanide, 18.9 g/l ofKBr and a pH of 5.2.

g/l of potassium ferricyanide and 1 g/l of potassium bromide were addedto make up the concentration. The bleaching effect of the reoxidised andregenerated bleaching bath was excellent.

Although the invention has been discribed in connection with thepotassium ferricyanide bleaching bath, it is to be understood that alsoother useful bleaching agents such as other iron lll complexes ofcomplex-free iron 11] salts as well as copper ll salts can berejuvenated continuously or batchwise according to the inventionprovided they are not decomposed by the regenerator solution. In thecase of iron lIl-salts or copper 11 salts it is advisable to change thepH to lower values in order to avoid hydrolysis and precipitation of thehydroxides.

We claim:

1. A method of regenerating a used photographic silver bleaching bathcontaining an alkali metal ferricyanide, an alkali metal ferrocyanideand an alkali metal halide, which comprises continually measuring theredox potential of the bleaching bath, thereby sensing the concentrationof ferrocyanide formed in the bleaching bath by reduction offerricyanide simultaneously with the bleaching of silver in thephotographic material, producing a halogen by oxidation of halide ionsin an aqueous solution in a closed vessel upon reaction betweenhydrohalic acid and an oxidizing agent to completely decompose the acidand form the halogen, immediately delivering the free halogen from theclosed vessel into the bleaching bath under the surface of the bath,variably controlling the input of the hydrohalic acid and the oxidizingagent into said reaction in the closed vessel in response to the redoxpotential in the bleaching bath, whereby the quantity of halogenproduced in the vessel is equivalent to the concentration offerrocyanidc in the bleaching bath and reacting the halogen with theferrocyanide to essentially restore the original concentration offerricyanide and halide in the bleaching bath.

2. A method as in claim 1 wherein the halogen is bromine formed in acidaqueous solution by oxidation of bromide ions.

3. A method as in claim 2 wherein the bromine is formed by reaction ofan alkali metal bromate with hydrobromic acid.

4. A method as in claim 2 wherein the bromine is formed by reaction ofan alkali metal bromate with an alkali metal bromide and hydrochloricacid.

5. A method as in claim 2 wherein the bromine is formed by reaction ofan alkali metal bromate with an alkali metal bromide and orthophosphoricacid.

6. A method as in claim 2 wherein the bromine is formed by reaction ofhydrogen peroxide with hydrobromic acid.

7. A method as in claim 1 wherein the bleaching bath is continouslyremoved from the processing tank, reoxidized while in through-flow andreturned to the tank by a pump circulatory system.

8. In the method as in claim 1 wherein the photographic material istreated in one or more additional processing baths, and theconcentration of active substances in said additional processing bathsis varied essentially in proportion to the quantity of elementary silverconverted to a silver salt in said bleaching bath, the improvementaccording to which the respective additional baths are regenerated byvariably controlling the input of the respective regenerating substancesinto the respective baths in response to the redox potential in thebleaching bath and in proportion to the quantity of halogen produced inthe closed vessel and delivered to the bleaching bath.

Patent No. '3 Dated Oct '1 I 1971!.

lnventofls) Wolfgang Hunicke et a1 d that error" appears in theabove-identified patent It is certifie cted. as shown belowzand thatsaid Letters Patent az e hereby corre Front page, line 10 of,Ab.stract,":60" should be of a I Column 2, lines 29-30, "In this way bromine isonly surface of the bleaching bath." should-be deleted. Column l, line16, "15" should be 105 Column 9, line 33, "of" should be-- or Signed ands'ealedhthis 11th day of February 1975.1

(SEAL) Attest: I

, I C. MARSHALL DANN RUTH C. MASON I Commissioner of Patents v,Attesting Officer 1 and Trademarks

1. A METHOD OF REGENERATING A USED PHOTOGRAPIC SILVER BLEACHING BATHCONTAINING AN ALKALI METAL FERRICYANIDE, AN ALKALI METAL FERROCYANIDEAND AN ALKALI METAL HALIDE, WHICH COMPRISES CONTINUALLY MEASURING THEREDOX POTENTIAL OF THE BLEACHING BATH, THEREBY SENSING THE CONCENTRATIONOF FERROCYANIDE FORMED IN THE BLEACHING BATH BY REDUCTION OFFERRICYANIDE SIMULTANEOUSLY WITH THE BLEACHING OF SILVER IN THEPHOTOGRAPHIC MATERIAL, PRODUCING A HALOGEN BY OXIDATION OF HALIDE IONSIN AQUEOUS SOLUTION IN A CLOSED VESSEL UPON REACTION BETWEEN HYDROHALICACID AND AN OXIDIZING AGENT TO COMPLETELY DECOMPOSE THE ACID AND FORMTHE HALOGEN, IMMEDIATELY DELIVERING THE FREE HALOGEN FROM TE CLOSEDVESSEL INTO THE BLEACHING BATH UNDER THE SURFACE OF THE BATH, VARIABLECONTROLLING THE INPUT OF THE HYDROHALIC ACID AND THE OXIDIZING AGENTINTO SAID REACTION IN THE CLOSED VESSEL IN RESPONSE TO THEREDOXPOTENTIAL IN THE BLEACHING BATH, WHEREBY THE QUANTITY OF HALOGENPRODUCED IN THE VESSEL IS EQUIVALENT TO THE CONCENTRATION OFFERROCYANIDE IN THE LEACHING BATH AND REACTING THE HALOGEN WITH THEFERROCYANIDE TO ESENTIALLY RESTORE THE ORIGINAL CONCENTRATON OFFERRICYANIDE AND HALIDE IN THE BLEACHING BATH.
 2. A method as in claim 1wherein the halogen is bromine formed in acid aqueous solution byoxidation of bromide ions.
 3. A method as in claim 2 wherein the bromineis formed by reaction of an alkali metal bromate with hydrobromic acid.4. A method as in claim 2 wherein the bromine is formed by reaction ofan alkali metal bromate with an alkali metal bromide and hydrochloricacid.
 5. A method as in claim 2 wherein the bromine is formed byreaction of an alkali metal bromate with an alkali metal bromide andorthophosphoric acid.
 6. A method as in claim 2 wherein the bromine isformed by reaction of hydrogen peroxide with hydrobromic acid.
 7. Amethod as in claim 1 wherein the bleaching bath is continously rEmovedfrom the processing tank, reoxidized while in through-flow and returnedto the tank by a pump circulatory system.
 8. In the method as in claim 1wherein the photographic material is treated in one or more additionalprocessing baths, and the concentration of active substances in saidadditional processing baths is varied essentially in proportion to thequantity of elementary silver converted to a silver salt in saidbleaching bath, the improvement according to which the respectiveadditional baths are regenerated by variably controlling the input ofthe respective regenerating substances into the respective baths inresponse to the redox potential in the bleaching bath and in proportionto the quantity of halogen produced in the closed vessel and deliveredto the bleaching bath.